It is well known that microwave energy propagated into a conductive cavity sets up a complex standing wave which is characterized by a large number of "cold" and "hot" spots. Early development microwave ovens accordingly exhibited very nonuniform heating.
Through the years, significant advances have been made in pattern uniformity. The first important step was the use of a mode stirrer which is merely a moving conductive device which alters the standing wave pattern. Later, rotating directive microwave radiators provided increased uniformity. However, even with these improved feed structures, it still may be desirable to stop the oven at some point during the cooking cycle and physically move the food within the cavity.
Another prior art attempt to improve heating uniformity was the introduction of a support surface in the oven cavity which moved. The common approach was to have a rotating turntable. Next, a portable turntable was introduced to the market place; it is described in U.S. Pat. No. 4,258,630 issued Mar. 31, 1981. However, there are still significant disadvantages. For example, the described portable turntable has a relatively high profile which substantially reduces the available cooking space in a microwave oven cavity. Also, the rotating surface area is limited because there is only one food support surface. Further, the relatively high profile would be even more disadvantageous if there were a second level.